Sheikh `Alî A. al-Jumu`ah, professor at al-Imâm University, Qasîm Branch
Combining between two prayers is permissible for travelers while they are actually in the act of traveling, beginning from the time they actually start their journey. This is allowed in order to facilitate and ease their travel. It is unlawful for residents, regardless of their duration of stay in a locality.
The Zuhr and `Asr prayers can be combined at any time from the start of the time for the Zuhr prayer until the end of the time of the `Asr prayer. The Maghrib and `Ishâ’ prayers can be combined from the start of the time of the Maghrib prayer until the end of the time of the time of the `Ishâ’ prayer.
Shortening the prayer, on the other hand, is permissible as well as being a Sunnah for both travelers and people staying in a locality temporarily. I hold the view that the maximum period of time for a person to remain in one place and still be considered a traveler is four days. If a person stays more than four days in one locality, he should not shorten his prayers. There are, however, a number of other viewpoints on this matter.
Many people confuse the issue of combining prayers with that of shortening them. They have the misconception that the two always go together, while this is not the case.
Source: Islam Today